The present invention relates to a capacitive liquid level sensor, and more particularly, to a compensated capacitive liquid level sensor, which is insensitive to the dielectric constant of the liquid, the level of which is to be sensed. The resulting dielectric-constant-insensitive capacitive liquid level sensor has the advantage that prior knowledge of the liquid, the level of which is to be sensed, is not required. I.e., a particular vessel and sensor and sensing electronics may be provided, and used for virtually any desired liquid.
Further, the liquid-level sensor of the present invention may also be employed as an inclinometer, measuring the tilt angle relative to gravity.
1. General Description
It was found by the inventor that a compensated capacitive liquid level sensor can be made, of the fringing-field type, i.e., where the capacitive plates are co-planar, and the effective xe2x80x9careaxe2x80x9d of the plates is essentially zero and the liquid interacts with the fringing field between the electrodes, as shown in FIG. 2. This approach can be implemented on a flat substrate, such as a rigid or flexible printed circuit board, with the resulting two advantages:
1. Simple, batch, manufacturing.
2. The processing electronics can be mounted on the same board, thus saving interconnection wiring and reducing sensitivity to external interferences.
3. The sensor can be made as a rigid printed circuit board and can be snap mounted, e.g., on the fuel pump assembly in an automotive gas tank.
Another aspect of the present invention is a unique geometry for the capacitive plates which enables a measurement range extending the whole sensor length. This geometry provides two capacitances each extending the whole sensor length, and whose ratio is directly proportional to the liquid level, independent of the dielectric constant of the liquid. The geometry is also applicable to uniform field sensors.
Another object of the invention is a capacitive level sensor for automotive application that is cost competitive with float-type level sensors.
Another object of the invention is a fringing-field capacitive level sensor that is relatively insensitive to contamination deposited between the electrodes of the fringing-field capacitive sensor, and relatively insensitive to liquid splashed over the non-immersed portion of the sensing plates. Such a sensor is typically useful for automotive applications.
In an inclinometer application, the tilt angle relative to gravity may be measured, providing an output proportional to the sine of the tilt angle.
In the following description and claims the terms plates and electrodes are interchangeably used with respect to conductive elements of the capacitors. Also the term xe2x80x9cuniform field capacitive sensorxe2x80x9d and xe2x80x9cparallel plates capacitive sensorsxe2x80x9d are interchangeably used and include two facing capacitor plates not necessarily planar and not necessarily with a uniform spacing.
2. Prior Art
There are various methods for measuring the liquid level in tanks. A survey of these methods can be found in: A Look at Level Sensing Sensors, pp. 29-34, August 1990, Vol. 7, No. 9. One well known method is based on the effect of the liquid level on capacitance plates, wherein capacitive plates interacting with the liquid are excited with an alternating voltage to generate a signal current depending on the liquid level.
The basic capacitive level sensor gives an erroneous result if the dielectric constant of the liquid is other than the design value, or is unknown. Compensation for variations in the dielectric constant can be performed as long as two capacitors are utilized because both capacitors will have capacitances dependent on the dielectric constant.
Prior art capacitive liquid level sensors, whether compensated or not, can be divided into uniform-field type and fringing-field type. The first type employs either parallel plates as in FIG. 1B or concentric cylinders as in FIG. 1A. In these capacitors, it is the parallel (or radial) field between the plates that interacts with the liquid, wherein the fringing field at the edges is insignificant in comparison to the parallel or radial field. The advantage of these two configurations is that by controlling the separation between the plates, the capacitance per unit height can be made relatively large, providing an accordingly large signal current. In the fringing-field type of capacitive sensor the electric field that interacts with the liquid is non-uniform.
Prior art compensated capacitive level sensors generally include an additional, known, reference capacitor that is totally immersed in the liquid. This reference capacitor is intended for measuring the dielectric constant of the liquid. By dividing the measured value of the measurement capacitance by that of the reference capacitor, a normalized output is obtained that is independent of the dielectric constant of the liquid. Level sensors of this type are described, for example, in U.S. Pat. Nos. 4,590,575, 4,667,646, 4,373,389, 4,296,630, and in 4,021,707.
An example of a prior art sensor with a reference capacitor at the bottom of the measurement vessel is provided by McDonald, U.S. Pat. No. 5,050,431, which uses a reference capacitor that is totally immersed in the liquid to compensate for variations in the dielectric constant. An interleaved area below the lower end of an electrode provides for a reference area.
In the following, the term level sensor will refer to compensated capacitive level sensors. The disadvantage of prior art level sensors, which are invariably of the uniform-field type, are the following:
1. Each of the two capacitors comprises spaced-apart plates, so the construction is relatively complicated since the plates must be isolated electrically, secured mechanically, and wired to the processing electronics.
2. Since the reference capacitor must be totally immersed in the liquid, an erroneous output is obtained for liquid levels lower than the reference capacitor height.
3. The practical requirement that the reference capacitor be located at the bottom of the container makes the measurement sensitive to contaminant liquids at the bottom of the container, since the measured dielectric constant is no longer representative of the intended liquid, for example water in a gasoline tank.
Various attempts have been made to provide a capacitive liquid level sensor without the problem of the reference capacitor at the bottom of the measurement vessel.
U.S. Pat. No. 4,373,389 is intended to overcome the deficiencies of a reference capacitor at the bottom of the container. In this patent there are two capacitive plates, one of them divided into two complementary triangles, resulting in two capacitors, each responding to the liquid level starting from bottom of the container to the maximum level to be measured. By manipulating the measured values of the measured two capacitances, the liquid level can be determined regardless of its dielectric constant. However, since the calculation is not simple, an analog-to-digital conversion, a microprocessor, and, possibly, a digital-to-analog conversion are necessary.
Lee, in U.S. Pat. No. 5,423,214, uses multiple discrete capacitors at discrete levels. However, the sensor provides a stepwise response to liquid level, rather than a continuous response.
German Patent Application DE 42 10737 describes an automotive fuel level sensor that employs the fringing field lines of a co-planar pair of electrodes that are applied on the outside of the tankxe2x80x94which must be non-metallic in order for the lines to penetrate into the tank. However, such sensor has a limited performance because of the following reasons:
1. Since the fuel is separated from the electrodes by the thickness of the plastic wall (∈ approximately 4), most of the field lines of force are shunted by the wall, and the sensor may respond unpredictably to the wall thickness, rather than to the presence of the fuel (∈ approximately 1.8).
2. The DE invention attempts to correct this deficiency by independently sensing the dielectric constant of the wall, however, no compensation is made for the thickness variations of the wall.
3. No compensation is made for dielectric constant variations of the fuel such as that result from additions such as of alcoholic additives.
4. The electrodes being on the outside of the tank may respond to soil deposits that adhere to the electrodes, and to fuel splashed on the non-immersed portions of the electrodes.
Compensation for the tilt angle relative to gravity of the container holding the liquid can be accomplished with the addition of a third capacitor. Florentin et al., in U.S. Pat. No. 4,935,727, use three spaced apart separate fuel sensors, for measuring the quantity of liquid obtained in a tank, which is subject to the least possible variation with changes in attitude and state of movement of the tank itself. However, the three fuel sensors are spaced angularly apart by 120xc2x0. In addition, Florentin et al. require an additional circuit to perform compensation for electrical liquid properties, even when sensing capacitors are used for the level sensors.
There is thus a widely recognized need for, and it would be highly advantageous to have, a capacitive liquid level sensor which is independent of the dielectric constant of the liquid, and preferably compensating also for a tilt angle, with a measurement range that extends the full length of the sensor plates, with a simple construction, and providing a linear output with no recourse to complex electronic manipulations, and which does not depend on the use of a fully-submerged reference sensor to achieve a reading which is independent of the dielectric constant of the liquid.
It was found that a compensated capacitive liquid level sensor can be made where the capacitive plates are co-planar, i.e., the effective xe2x80x9careaxe2x80x9d of the plates is essentially zero and the liquid interacts with the fringing field as shown in FIG. 2.
Preferably, the sensor is based on a thin non conductive substrate coated with conductive capacitive plates so that it is sensitive to even the low dielectric constant of fuel like gasoline (∈ approximately 1.8), but is relatively insensitive to contamination deposited on the electrodes and to the substrate material itself.
Another aspect of the present invention is a unique geometry for the capacitive plates, which extends the whole measurement range.
Another aspect of the invention described below, is that the liquid level sensor does not require the use of a fully-immersed full-scale reference capacitor as in the prior art. In the prior art, for compensated liquid level sensors, a voltage proportional to the capacitance of a capacitor with a constant incremental capacitance is divided by a voltage proportional to the capacitance of a fully-immersed referenced capacitor, i.e., with a zero incremental capacitance. In the current invention, both capacitors preferably have non-zero incremental capacitances as will be explained below.
Another object of the invention is a capacitive level sensor for automotive application that is cost competitive with float type level sensors.
Another aspect of the current invention involves the use of two spaced apart level sensors wherein by taking the difference of their two outputs a signal proportional to the tilt angle of the liquid tank is obtained in addition to the liquid level.
Accordingly, there is provided a uniform field compensated-type capacitive sensor for measuring the liquid level of a liquid having a dielectric constant inside a container, wherein the sensor is fixed to the container and includes: first and second non-conductive substrates facing each other; at least three electrodes printed on the substrates that interact capacitively with the liquid, the at least three electrodes constituting at least two sensing capacitors with capacitances which depend on the liquid level, wherein the electrodes extend substantially from a lower level to a higher level of the container, the lower level and the higher level bounding a desired measurement range, and wherein the functions that describe the dependence of the capacitances on the liquid level differ from each other; and signal processing means for converting the capacitances into electrical signals that can be processed to provide an output signal indicative of the liquid level relative to the sensor, wherein the output signal is substantially independent of the electrical properties of the liquid.
According to certain preferred embodiments, each of the at least two sensing capacitors has an incremental capacitance, and a ratio of the incremental capacitances integrated from the lower level to the liquid level is proportional or linearly proportional to the liquid level and substantially independent of the electrical properties of the liquid.
According to still further features of preferred embodiments of the current invention, the sensor further includes: a sealed enclosure, with at least a portion of the signal processing means contained within the enclosure.
In embodiments where the container has a tilt angle relative to gravity in at least one axis, the sensor preferably further includes: at least one other sensing capacitor with an output signal, wherein the at least one other sensing capacitor is spaced apart from the at least three electrodes. Preferably, the signal processing means further includes electronics for processing the output signal from the at least one other sensing capacitor, and the output signal from the signal processing means is also substantially independent of the tilt angle of the container.
According to a second preferred embodiment of the current invention, there is provided a fringing field capacitive sensor for measuring the liquid level of a liquid having a dielectric constant inside a container, wherein the sensor is fixed to the container and comprises: a non-conductive substrate with at least a first exposed surface; at least three electrodes printed on the substrate that interact capacitively with the liquid, the at least three electrodes constituting at least two sensing capacitors with capacitances which depend on the liquid level, wherein the electrodes extend substantially from a lower level to a higher level of the container, the lower level and the higher level bounding a desired measurement range, and wherein the functions that describe the dependence of the capacitances on the liquid level differ from each other; and signal processing means for converting the capacitances into electrical signals that can be processed to provide an output signal indicative of the liquid level relative to the sensor, wherein the output signal is substantially independent of the electrical properties of the liquid.
According to certain preferred embodiments, each of the at least two sensing capacitors has an incremental capacitance, and a ratio of the incremental capacitances integrated from the lower level to the liquid level is proportional or linearly proportional to the liquid level and substantially independent of the electrical properties of the liquid.
In certain preferred embodiments, the substrate comprises a flexible plastic sheet. Preferably, the substrate is a Mylar sheet and the electrodes are printed with carbon ink. Preferably, the electrodes are interdigitated and slanted.
According to still further features in preferred embodiments of the current invention, the sensor further includes: a shield electrode printed between electrodes of at least one of the at least two sensing capacitors for decreasing the sensitivity of the output signal to contamination and liquid splashes on the substrate.
Preferably, the sensor further includes: a sealed enclosure, wherein at least a portion of the signal processing means are contained within the enclosure. The enclosure preferably also encloses capacitor plates printed on the substrate and enabling compensation for temperature induced variations to the capacitances.
According to certain preferred embodiments, the at least two sensing capacitors are printed on the first exposed surface of the substrate, and a grounded layer is printed on a second exposed surface of the substrate to protect against external electrical interference.
In embodiments where the container has a tilt angle relative to gravity in at least one axis, the sensor preferably further includes: at least one other sensing capacitor with an output signal, wherein the at least one other sensing capacitor is printed on the substrate and spaced apart from the at least three electrodes. Preferably; the signal processing means further includes electronics for processing the output signal from the at least one other sensing capacitor, and the output signal from the signal processing means is also substantially independent of the tilt angle of the container.